Characterization of the Chemical Behavior of the Low Excited States through a Local Chemical Potential

Exploiting the locality of the chemical potential of an excited state when it is evaluated using the ground-state density functional theory (DFT), a new descriptor for excited states has been proposed. This index is based on the assumption that the relaxation of the electronic density drives the chemical reactivity of excited states. The sign of the descriptor characterizes the electrophilic or nucleophilic behavior of the atomic regions. A relation between the new descriptor and the dual descriptor is derived and provides a posteriori justification of its use to rationalize the Woodward−Hoffmann rules for photochemical reactions within the conceptual DFT. Finally, the descriptor is successfully applied to some [2 + 2] photocycloadditions, like Paterno−Büchi reactions.